The present invention relates generally to an electrical switching device, and more particularly to, a method and apparatus to prevent contact welding subsequent to variable fault current conditions in an electromagnetic contactor.
Electromagnetic contactors are used in starter applications to switch on/off a load as well as to protect a load, such as a motor, from current overloading. Contactors are used as electrical switching devices and incorporate fixed and movable contacts that when closed, conduct electric power. Once closed, the contacts are biased toward one another. A well-known problem with contactors having contacts biased together is the welding of the contacts during the occurrence of a short circuit event.
There are several known methods of preventing contact welding in electrical switching devices such as an electromagnetic contactor. One method is the selection of composite materials for the contacts that resist welding under low fault current conditions. Generally, contacts can be blown open due to a magnetic constriction force that is greater than a bias spring force that normally holds the contact closed. An arc forms across the contacts as soon as the contacts part. This arc energy can melt the contact surface and when the contacts re-close when the bias spring force exceeds the dissipating constriction force before current zero, the contacts can weld together. The contacts blow open even at low fault currents, but they do not form weld or only extremely light weld due to weld resistance of the contact material. Due to the chemical composition and the physical structure, composite contact materials can prevent welding of the contacts, and in some cases, can withstand light welding during low fault current events. These light welds can easily be broken by the opening force of the contactors when switched open.
Another method available for intermediate fault current conditions incorporates magnetic components within a contact carrier wherein the magnetic components are in operable association with the contact carrier to keep the contacts apart for a period of time after a fault. Because of the low thermal resistances and high melting points, the contact materials solidify rapidly after melting due to rapid cooling by convection, radiation and conduction. Thus, preventing contact closure for a short time duration after passage of the arc current through the contacts can provide sufficient time for the contacts to harden and not weld together. Such prior art devices disclose magnetic components that influence the biasing forces on the contacts thereby delaying the time of contact closure to permit cooling of the surfaces of the contacts.
Another method of assisting in preventing contact welding is through forced opening of the contactors under high fault currents. A short circuit fault current generates extremely high arc pressure across the contact surfaces in the contactor. This arc pressure can be directed to overcome the magnetic force generated by the armature and the magnetic coil to open the contactor.
Each of the above mentioned methods for the prevention of contact welding have certain drawbacks and limitations. For example, utilizing a contact material that is resistant to welding is feasible during low fault current conditions, but not intermediate to high fault currents. Under intermediate fault currents, magnetic components can be utilized to provide additional time after current zero before contact re-closing, however, often reduced space requirements for the contactor require smaller magnetic components for the magnetic latching function resulting in a saturation effect at fault currents well below a peak current value. The saturation effect causes the magnetic force created by the magnetic components to increase linearly instead of exponentially, which limits the effectiveness of the magnetic latching to prevent contact welding. Likewise, blow open during high fault currents, combined with the increased force created by the biasing spring when further compressed, closes the contacts before the contacts have been cooled sufficiently, thereby causing the contacts to weld together.
Therefore, it would be desirable to have an electromagnetic contactor capable of withstanding a myriad of fault currents that is adaptable for various physical dimensions of the contactor. Such a contactor would prevent welding of the contacts under low fault current conditions, intermediate fault current conditions, and high fault current conditions.
The present invention provides a system and method of preventing welding between the movable and stationary contacts in an electromagnetic contactor that overcomes the aforementioned drawbacks and provides a device that operates within a wide range of fault current values. The contactor prevents welding of the contacts under low fault current conditions by fabrication of the contacts using a weld resistant material, under intermediate fault current conditions by utilization of magnetic components to temporarily latch the contacts in an open position until the fault current dissipates and the contacts solidify, and under high fault current conditions by preventing the contacts from re-closing upon themselves until the contactor is reset.
The invention includes a contactor having stationary and movable contacts biased towards each other and switchable between an open and a closed position. Energization of an electromagnetic coil engages the contacts creating an electric path for current flow through the contactor. An electromagnetic coil is used that allows the use of a lower holding power once engaged. The invention uses pulse modulation after the contactor is initially engaged to maintain the contactor in a closed position. The contacts may be disengaged and then reset to a contact closed position by spring biasing under low and intermediate fault current conditions, without contact welding with the use of specialized contact material and with the use of magnetic components to compensate for low and intermediate fault currents, respectively. A high fault current creates a blow open effect wherein the armature separates from the electromagnetic coil and disengages the stationary and movable contacts permanently until application of a second energizing pulse to the electromagnetic coil at or above an activation threshold level.
In accordance with one aspect of the present invention, a contactor comprising a contactor housing with stationary contacts mounted within the housing and a contact bridge having movable contacts mounted to the bridge is disclosed. A movable contact carrier is slidably mounted within the contactor housing and has a biasing mechanism between the contact bridge and the movable contact carrier to bias the contact bridge and the movable contacts toward the stationary contacts. An armature is secured to the movable contact carrier and drawn into an electromagnetic coil mounted in the contactor housing thereby closing the movable contacts onto the stationary contacts when the coil is energized by a first energy source. A second energy source, lower than the first energy source, maintains the armature within the electromagnetic coil until released or the occurrence of a high fault current. A high fault current creates a high arc pressure across the contacts within an arc pressure containment mechanism situated about the stationary and movable contacts to disengage the armature from the electromagnetic coil and open the movable contacts from the stationary contacts until the first energy source is reapplied to the electromagnetic coil.
Yet another aspect of the present invention includes a variable fault current tolerable contactor comprising a contactor housing with a stationary contact therein and a contact carrier movable within the contactor housing. A movable contact mounted within the movable contact carrier and in operable association with the stationary contact is switchable between an open position and a closed position, and while in the closed position, allows electrical current to flow through the stationary and movable contacts. An armature is attached to the movable contact carrier and a movable contact biasing mechanism is located between an upper enclosure of the movable contact carrier and the movable contact to bias the movable contact toward the stationary contact. An armature biasing mechanism is located between the armature and a base portion of the contactor housing to bias the armature towards the stationary contact. An electromagnetic coil is mounted in the contactor housing. The coil has an activation power threshold that once attained attracts the armature into the coil thereby engaging the movable contact with the stationary contact, and a reduced holding power threshold to maintain engagement of the contacts thereafter. Under a high fault current, an arrangement is provided wherein the reduced power threshold is overcome to disengage the armature from the electromagnetic coil to open the contacts until regeneration of the activation power threshold. The contactor then stays open until reset with an energizing pulse.
According to another aspect of the invention, a method to prevent contact weld is disclosed. The method includes providing a pair of contacts comprised of a weld resistant material, wherein the contacts are movable between a closed position and an opened position with respect to the other contact. An electromagnetic coil is energized with a first power source to create an electrical path through the pair of contacts when the contacts are in the closed position. Under intermediate to high fault current conditions, the contacts are opened due to a high constriction force on the surface of the contacts. Under intermediate fault currents, the contacts remain open temporarily after the fault current dissipates to provide sufficient time to cool which thereby prevents a welding of the contacts. By physically varying the distance between two magnetic components, the delay time until contact closure can be adjusted. After a high fault current, the contacts are blown open and remain in an open position until the first energy source is reapplied to the electromagnetic coil to overcome the activation power threshold and draw the contacts together.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.